Semiconductor gas sensor device and manufacturing method thereof

ABSTRACT

A semiconductor gas sensor device includes a first cavity that is enclosed by opposing first and second semiconductor substrate slices. At least one conducting filament is provided to extend over the first cavity, and a passageway is provided to permit gas to enter the first cavity. The sensor device may further including a second cavity that is hermetically enclosed by the opposing first and second semiconductor substrate slices. At least one another conducting filament is provided to extend over the second cavity.

PRIORITY CLAIM

This application claims priority from Italian Application for Patent No.MI2014A001186 filed Jun. 30, 2014, the disclosure of which isincorporated by reference.

TECHNICAL FIELD

The present disclosure relates to a semiconductor gas sensor device andthe manufacturing method thereof.

BACKGROUND

The thermal conductivity detector (TCD) is well known in the state ofthe art. A TCD is an environmental sensor device widely used for themeasurement of the amount of gas in the environment. The operation isbased on the fact that each gas has an inherent thermal conductivity anda filament (thermal resistor) changes its temperature as a function ofthe amount of gas that surrounds it. The most appropriate sensingelement shape is that of a thin finger suspended, for which thetemperature of the central part can locally reach even values of severalhundred degrees. The feature that the finger is totally suspended allowsfor enhancing the amount of heat exchange with the gas in which it isimmersed. The warming effect of the suspended finger is induced throughan electrical stress of the sensor, for example by means of the flow ofcurrent through the finger. The sensor is able to better discriminatethe gases whose conductivity is much different than normal air (roughlyN₂ (79%), O₂ (19%), CO₂ (0.04%), plus other gases with negligiblequantities: for example the CO is a few ppm).

When a current flows through the finger, the value of the resistance ofthe finger changes. The measurement of the resistance value allows formeasuring the conductivity of the gas mixture which depends on the molarfraction of the gas of interest.

However, it is difficult in principle to discriminate which is the gasmainly responsible for the conductivity variation of the mixture of gas.For example, CO₂ has a lower thermal conductivity than dry air,therefore if its percentage increases inside the mixture, this willraise the temperature of the sensor with a consequent increase of thevalue of the measured resistance.

The TCD sensor operates in accordance with the thermodynamic equilibriumamong heat generated by the current flow, heat exchange with thematerial of which the sensor is made (e.g., polysilicon crystalline),and heat exchange with the gas mixture surrounding it. The ambienttemperature determines the equilibrium value of the sensor in standarddry air. To take into account and compensate for the variation ofambient temperature, a Wheatstone bridge could be used as the sensorstructure. The reference branches of the bridge are of the same natureand positioned in the vicinity of the sensor so as to be sensitive tothe same way to changes in ambient temperature, with the difference thatwill not be exposed to the mixture of gas as the sensor.

The Relative Humidity (RH) is the amount of water vapor (gas) present inthe environment compared to a saturated environment in the sameconditions of pressure and temperature. The thermal conductivity ofwater vapor is much larger than the dry air therefore an increase inrelative humidity produces a lowering of the temperature of the sensorwith a consequent reduction of the value of the measured resistance. Thecontribution of the RH value of the measured resistance could be 1/10compared to the change of resistance in the presence of CO₂, therefore,this is a parameter to measure and correct. Typically the correction ismade by means of a dedicated sensor for the measurement of the RH.

In view of implementation of space saving and low power consumption, ademand exists to further reduce the size of gas detectors for measuringthe concentration of gas. In recent years, gas detection elements withgreatly reduced sizes have been developed by the use of MEMS(Micro-Electro-Mechanical System) technology (also called themicromachining technique). A gas detection element formed by use of MEMStechnology is configured such that a plurality of thin films are formedin layers on a semiconductor substrate (e.g., a silicon substrate).Examples of such a gas detection element include athermal-conductivity-type gas detection element. Thethermal-conductivity-type gas detection element has a heat-generatingresistor and utilizes the phenomenon that, when the heat-generatingresistor is energized and generates heat, heat is conducted to the gas.The conduction of heat causes a change in temperature of theheat-generating resistor and thus a change in resistance of theheat-generating resistor. On the basis of the amount of the change, thegas is detected. In the thermal-conductivity-type gas detection element,the resistance of the heat-generating resistor varies with the type orconcentration of the gas.

SUMMARY

One aspect of the present disclosure is to provide a semiconductor gassensor device of simple architecture with respect to the known ones.

One aspect of the present disclosure is a semiconductor gas sensordevice comprising: one doped semiconductor substrate of a firstsemiconductor slice, a first insulating layer placed above said dopedsemiconductor substrate, a part of at least one first cavity formedinside said first insulating layer and said doped semiconductorsubstrate and extending inside said doped semiconductor substrate to aprefixed depth, at least one conductive filament placed over said partof the at least one first cavity in a bridge way, a conductive metallayer placed at the ends of at least one filament for contact it,another doped semiconductor substrate of a second semiconductor slice,said another doped semiconductor substrate comprising the other part ofthe at least one first cavity and being placed above said dopedsemiconductor substrate of the first semiconductor slice so as to formsaid at least one first cavity and close it, said another dopedsemiconductor substrate comprising at least one hole in correspondenceof the first cavity for the inlet of gas to detect.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present disclosure, a preferredembodiments thereof are now described, purely by way of non-limitingexamples and with reference to the annexed drawings, wherein:

FIG. 1A shows a block diagram of a measurement apparatus comprising ameasurement device and an integrated gas sensor device according to afirst embodiment of the present disclosure;

FIG. 1B shows a block diagram of a measurement apparatus comprising ameasurement device and an integrated gas sensor device according to asecond embodiment of the present disclosure;

FIG. 2A shows a schematic layout of the integrated semiconductor gassensor device according to a second embodiment of the presentdisclosure;

FIG. 2B shows a schematic layout of the sensing elements of theintegrated semiconductor gas sensor device in FIG. 2A;

FIGS. 3 and 4 are cross sectional views of a part of the semiconductorgas sensor device according to the first embodiment of the presentdisclosure;

FIGS. 5-17 show the manufacturing process of the part of thesemiconductor gas sensor device in FIGS. 3 and 4;

FIGS. 18-19 show the manufacturing process of the other part of thesemiconductor gas sensor device according to the first embodiment of thepresent disclosure;

FIG. 20 is a cross sectional view more in detail of the semiconductorgas sensor device according to the first embodiment of the presentdisclosure.

DETAILED DESCRIPTION

FIG. 1A and FIG. 1B show a block diagram of a measurement apparatuscomprising a gas sensor device 1 or a gas sensor device 50 accordingrespectively to a first and a second embodiment of the presentdisclosure and a measurement device 100.

According to a first embodiment of the present disclosure, theintegrated semiconductor gas sensor device 1 comprises at least onevariable resistor R2 exposed to the gas (FIG. 1A). The terminals of theresistor R2 are connectable with a variable current generator 210 andground GND; also the terminals of the resistor R2 are connectable to themeasurement device 100 able to measure the voltage across the variableresistor R2. According to the present disclosure the resistor R2 isformed in a semiconductor substrate wherein at least a cavity 3 isperformed which is coated by silicon but is open to the outside by meansof one hole 35 so that the cavity 3 is exposed to the gases (FIG. 20).The resistor R2 is formed in the cavity 3 by means of suspended filament30 preferably in polysilicon; the filament 30 is arranged in a bridgeway. The sensitivity of the resistor R2 depends on the resistivity ofthe filaments 30, reducing the resistivity the sensitivity increases;for example a filament in polysilicon with size of 50×1×1 microns can beused.

According to a second embodiment of the present disclosure, theintegrated semiconductor gas sensor device is preferably a Wheatstonebridge 50 including a couple of reference resistors R1 and a couple ofvariable resistors R2 exposed to the gas (FIG. 1B); the use of aWheatstone bridge allows for minimizing the dependence on the ambienttemperature. The four connecting nodes A-D of the terminals of theresistances R1 and R2 of the Wheatstone bridge 50 are connectablerespectively with a variable current or voltage generator 210, to groundGND and to the measurement device 100 able to receive the voltage signalat the output of the Wheatstone bridge 50.

According to the present disclosure the Wheatstone bridge 50 is formedin a semiconductor substrate wherein two cavities 2 and 3 are formedwhich are both hermetically coated by silicon but wherein the onlycavity 3 is open to the outside by means of two holes 35 so that thecavity 3 is exposed to the gases (FIG. 2A). The couple of referenceresistors R1 are formed in the cavity 2 by means of suspended filaments20 while the couple of resistors R2 are formed in the cavity 3 by meansof suspended filaments 30 (FIG. 2B); the filaments 20 and 30 are formedpreferably in polysilicon. The sensitivity of the Wheatstone bridge 50depends on the resistivity of the filaments 30, reducing the resistivitythe sensitivity increases; for example a filament in polysilicon withsize of 50×1×1 microns can be used.

FIGS. 3, 4 and 20 show cross sectional views of a part of the integratedgas sensor device 1 formed in a semiconductor substrate; the sectionsshow a doped semiconductor substrate 12 of a first semiconductor slice40, preferably a silicon slice preferably of the n type, and at leastone insulating layers 10, 11 placed above said doped semiconductorsubstrate, but preferably a succession of a nitride 10 and oxide 11layers are placed over the semiconductor substrate 12.

A lower part 301 of the cavity 3 is formed inside said dopedsemiconductor substrate 12 and the at least one insulating layer 10, 11and extends inside said doped semiconductor substrate to a prefixeddepth Dp, for example of 10 microns; at least one conductive filament30, preferably made in polysilicon, is placed inside the cavity 3 in abridge way for forming the resistor R2, that is the conductive filament30 is suspended over the lower part 301 of the cavity 3. In the case ofthe integrated gas sensor device using a Wheatstone bridge 50, twofilaments 30 are placed inside the cavity 3 in a bridge way for formingthe resistors R2 and a lower part 201 of another cavity 2 is formedinside said doped semiconductor substrate 12 and the at least oneinsulating layer 10, 11 and extends inside said doped semiconductorsubstrate to the prefixed depth Dp, for example of 10 microns; twoconductive filaments 20, preferably made in polysilicon, are placedinside said lower part 201 of the cavity 2 in a bridge way for formingthe resistors R1, that is the conductive filaments 20 are suspended overthe lower part 201 of the cavity 2. The conductive filaments 20, 30 areplaced preferably at a distance of 100 micrometers.

A second insulating layer 15, preferably a nitride layer, is placedabove and around the at least one conductive filament 30 except in thecontact zones at the ends of the filament; a conductive metal layer 14is placed on the ends of the filament for contact them. In the case ofthe integrated gas sensor device using a Wheatstone bridge 50, thesecond insulating layer 15 is placed above and around said conductivefilaments 30, 20 except in the contact zones at the ends of eachfilament; a conductive metal layer 14 is placed on the ends of thefilaments for contact them so that the first pair of conductivefilaments 20 represent the reference resistors R1 of the Wheatstonebridge 50 while the second pair of conductive filaments 30 represent thevariable resistors of the Wheatstone bridge 50.

A doped semiconductor layer 501 of a second semiconductor slice 45 whichcomprises the upper part 302 of the at least one first cavity 3 isplaced above the first semiconductor slice 40 as to form the cavity 3and to close said cavity 3; the doped semiconductor layer 501 presentsat least one hole 35 for the inlet of gas to detect. In the case of theintegrated gas sensor device 50, the doped semiconductor layer 501comprises the upper part 302 of the cavity 3 and the upper parts 202 ofthe cavity 2; the doped semiconductor layer 501 is placed above thefirst semiconductor slice 40 as to form the cavities 2 and 3 and tohermetically close the cavity 2 and close the cavity 3. The dopedsemiconductor layer 501 present at least one hole 35 but preferably twoholes 35 for the inlet of gas to detect in the cavity 3.

The process for the formation of the semiconductor gas sensor device 1comprises a thermal oxidation of a part of a silicon substrate 12 of asfirst silicon slice 40, preferably an n type silicon substrate, forforming silicon oxide layers 11 over and under the substrate 12 with athickness of 0.5 micron and a deposition of insulating layers 10, forexample nitride layers with a thickness of 1000 angstrom, over the oxidelayers 11 (FIG. 5) formed over and under the silicon substrate 12 with athickness of 725 microns.

Successively a deposition of a polysilicon with a resistivity of 1.35mΩ×cm occurs over the upper nitride layer 10 for forming a conductivelayer 16, preferably a polysilicon layer 16, with a thickness of 1micron (FIG. 6).

The filament 30 or the filaments 20, 30 in the case of forming theWheatstone bridge 50 are then defined from the conductive layer 16 as isshown in FIG. 7; this is obtained by placing a lithographic mask overthe conductive layer 16 and successively effectuating a dry etching.

In the next step an insulating layer 15, preferably a nitride layer, isgrown above and around the polysilicon filament 30 or the polysiliconfilaments 20, 30 in the case of forming the Wheatstone bridge 50 andover the nitride layer 10 (FIG. 8, 9 wherein the section across thefilaments and the section along the filaments are respectively shown)and an activation step of the dopant of the polysilicon layer 16 iseffectuated.

Preferably successively RF sputtering deposition technique of metal, forexample a palladium, is effectuated over the nitride layer 15 forforming the palladium layer 17 which is defined by placing alithographic mask over the conductive palladium layer 17 andsuccessively effectuating an etching so that the palladium layer 17 ismaintained only over and around the polysilicon filament 30 or thepolysilicon filaments 20, 30 in the case of forming the Wheatstonebridge 50 (FIG. 10). The palladium layer 17 is optional and allowsprotecting the polysilicon filaments from weathering and making thepolysilicon filaments controllable by process and stable over the time.

Successively a silicon oxide deposition, preferably atetraethylorthosilicate (TEOS) silicon oxide, over the nitride layer 15for forming a silicon oxide layer 18 is effectuated (FIG. 11). Thethickness of the silicon oxide layer 18 is preferably of 3000 angstromsover the nitride layer 15 placed above the polysilicon filament 30 orthe polysilicon filaments 20, 30 in the case of forming the Wheatstonebridge 50.

The next step is a definition of the contact zones 31 of the filament 30or the contact zones 21, 31 (in the case of forming the Wheatstonebridge 50) of the filaments 20, 30 by placing a lithographic mask overthe layer 18 and successively effectuating a dry etching (FIG. 12) ofthe layers 15 and 18.

A RF sputtering deposition technique of metal, preferably Titanium andAluminum, is typically used for the formation of the metal layer 41(FIG. 13). A successive definition of the metal contacts is effectuatedby placing a lithographic mask over the layer 41 and successivelyeffectuating a dry etching (FIG. 13) of the layer 41. Resist strips 42are placed above the metal layer 41.

Then a deposition of a further silicon oxide layer 43 occurs, preferablyby means of two LPCVD depositions (low pressure chemical vapordeposition), with a thickness of preferably of 6000 angstroms, byobtaining total oxide layer of with a thicknesses preferably of 9000 and19000 angstroms over the nitride layer 15.

Successively the formation of the lower part 301 of the cavity 3 or thelower parts 201, 301 of the cavities 2 and 3 (in the case of forming theWheatstone bridge 50) occur. This is obtained by placing a lithographicmask over the layer 43 and successively effectuating a wet etching toarrive to the nitride layer 15 in the area around the filament 30 oreach filament 20, 30 (FIG. 14), placing another lithographic mask overthe nitride layer 15 and successively effectuating a dry etching of thenitride layers 15, 10 and the oxide layer 11 to arrive to the substrate12 always in the area around the filament 30 or each filament 20, 30preferably at a distance of 0.3 microns from the polysilicon filament 30or each polysilicon filament 20, 30 (FIG. 15) and placing a resist mask44 over the substrate 12 so that the resist layer is placed at adistance of 0.8 microns from the polysilicon filament 30 or eachpolysilicon filament 20, 30 and 0.5 microns from the oxide layer 11(FIG. 16) and successively effectuating an anisotropic and isotropic dryetching of the substrate 12 of a depth Dp=10 microns (FIG. 17) to etcheven the portion of the silicon substrate 12 under the from thepolysilicon filament 30 or the polysilicon filaments 20, 30.

Successively another dry etching of the oxide layer 11 occurs to arriveat the device in FIGS. 3, 4.

The process for the formation of the semiconductor gas sensor devicecomprises placing a resist mask on a semiconductor substrate 501 of asecond semiconductor slice 45, preferably a n type silicon substrate, todefine the upper part 302 of the cavity 3 or the upper parts 202, 302 ofthe cavities 2, 3 and successively effectuating an anisotropic andisotropic dry etching of the substrate 501 for a depth Dp=10 microns(FIG. 18) to form the upper part 302 of the cavity 3 or the upper parts202, 302 of the cavities 2, 3.

The process comprises even placing a resist mask 52 on a siliconsubstrate 50 after the definition of the upper part 302 of the cavity 3,to define at least one hole 35 only in the upper part 302 of the cavity3 and successively effectuating an anisotropic and isotropic dry etchingof the substrate 50 to form the hole 35 in the upper part 302 of thecavity 3 (FIG. 19).

The first 40 and second 45 semiconductor slices are joined together sothat the lower part 301 corresponds to the upper part 302 or the lowerparts 301, 201 correspond respectively to the upper parts 202, 302; inthis way the only cavity 3 or both the cavities 2 and 3 are formed.Preferably the first 40 and second 45 semiconductor slices are joinedtogether by using an adhesive 60 such as glass frit or dry resist (FIG.20).

1. A semiconductor gas sensor device, comprising: a doped semiconductorsubstrate of a first semiconductor slice, a first insulating layerplaced above said doped semiconductor substrate, a part of at least onefirst cavity formed inside said first insulating layer and said dopedsemiconductor substrate and extending inside said doped semiconductorsubstrate to a first depth, at least one conductive filament placed oversaid part of the at least one first cavity in a bridge way, a conductivemetal layer placed at the ends of at least one filament for makingelectrical contact, another doped semiconductor substrate of a secondsemiconductor slice comprising another part of the at least one firstcavity and being placed above said first semiconductor slice so as toform and close said at least one first cavity, said another dopedsemiconductor substrate comprising at least one hole in correspondenceof the first cavity for the inlet of gas to detect.
 2. The semiconductorgas sensor device according to claim 1, wherein the first semiconductorslice comprises: parts of the first cavity and a second cavity formedinside said first insulating layer and said doped semiconductorsubstrate and extending inside said doped semiconductor substrate to thefirst depth, a first pair of conductive filaments and a second pair ofconductive filaments placed inside said respective first and secondcavities in a bridge way, said first and second pairs of conductivefilaments being respectively the variable resistors and the referenceresistors of a Wheatstone bridge, the conductive metal layer beingplaced at the ends of each filament of the first and the second pairs ofconductive filaments for making electrical contact, said another dopedsemiconductor substrate comprising other parts of the first and secondcavities and being placed above said first semiconductor slice so as toform said first and a second cavities and hermetically close the secondcavity and close the first cavity, said another doped semiconductorsubstrate comprising at least one hole in correspondence only of thefirst cavity for the inlet of the gas to detect.
 3. The semiconductorgas sensor device according to claim 2, wherein said another dopedsemiconductor substrate comprises two holes in correspondence only ofthe first cavity for the inlet of the gas to detect.
 4. Thesemiconductor gas sensor device according to claim 1, wherein theintegrated gas sensor device comprises a second insulating layer placedabove and around said at least one conductive filament.
 5. Thesemiconductor gas sensor device according to claim 1, wherein the firstand second semiconductor slices are joined together by an adhesive. 6.The semiconductor gas sensor device according to claim 5, wherein theadhesive is glass frit.
 7. A method for manufacturing a semiconductorgas sensor device, comprising: forming a doped semiconductor substrateof a first semiconductor slice, forming a first insulating layer abovesaid doped semiconductor substrate, forming a part of at least one firstcavity inside said first insulating layer and said doped semiconductorsubstrate so that said part of at least one first cavity extends insidesaid doped semiconductor substrate to a first depth, forming at leastone conductive filament over said part of the at least one first cavityin a bridge way, forming a conductive metal layer at the ends of atleast one filament for making electrical contact, forming another dopedsemiconductor substrate of a second semiconductor slice, forming theother part of the at least one first cavity with said another dopedsemiconductor substrate, forming at least one hole in correspondence ofsaid other part of the at least one first cavity for the inlet of gas todetect, placing the second semiconductor slice above said firstsemiconductor slice so as to form and close said at least one firstcavity.
 8. The method according to claim 7, with respect to the firstsemiconductor slice, further comprises: forming parts of the firstcavity and a second cavity inside said first insulating layer and saiddoped semiconductor substrate so that said parts of the first and secondcavities extend inside said doped semiconductor substrate to the firstdepth, forming a first pair of conductive filaments and a second pair ofconductive filaments inside said respective first and second cavities ina bridge way, said first and second pairs of conductive filaments beingrespectively the variable resistors and the reference resistors of aWheatstone bridge, forming the conductive metal layer on the ends ofeach filament of the first and second pairs of conductive filaments formaking electrical contact, forming other parts of said first and secondcavities with said another doped semiconductor substrate, forming atleast one hole only in correspondence of said other part of the firstcavity for the inlet of gas to detect, placing the second semiconductorslice above said first semiconductor slice so as to form said first andsecond cavities and hermetically close the second cavity and close thefirst cavity.
 9. The method according to claim 8, with respect to theanother doped semiconductor substrate, further comprising forming twoholes in correspondence only of the first cavity for the inlet of thegas to detect.
 10. The method according to claim 7, further comprisingforming a second insulating layer above and around said at least oneconductive filament.
 11. The method according to claim 7, whereinplacing the first semiconductor slice over the second semiconductorslice comprises joining the first semiconductor slice and the secondsemiconductor slice using an adhesive.
 12. The method according to claim11, wherein the adhesive is glass frit.
 13. A sensor, comprising: afirst substrate including a first cavity; a first conductive filamentextending over said first cavity on a first bridge way; first conductivecontacts at each end of the first conductive filament; a secondsubstrate placed above said first substrate and configured to enclosesaid first cavity; and a passageway extending through at least one ofthe first and second substrates and into said first cavity forpermitting passage of a gas to be detected by the sensor.
 14. The sensorof claim 13, further comprising: the first substrate including a secondcavity; a second conductive filament extending over said second cavityon a second bridge way; second conductive contacts at each end of thesecond conductive filament; and the second substrate further configuredto hermetically enclose said second cavity.
 15. The sensor of claim 13,further comprising: a third conductive filament extending over saidfirst cavity on a third bridge way; a fourth conductive filamentextending over said second cavity on a fourth bridge way; the firstthrough fourth conductive filaments electrically connected to form aWheatstone bridge circuit.